A FRAM (Ferroelectric Random Access Memory device) or a DRAM (Dynamic Random Access Memory device) of a semiconductor device employs a capacitor with a transistor. The more a semiconductor device is integrated, the more a size of capacitor is forced to be narrow, and thereby deterioration may occur in a semiconductor device. Therefore, in a semiconductor device having a capacitor, there exists a need for designs and methods that can achieve high capacity in a very small capacitor.
As the integration density is higher, the use of a ferroelectic capacitor in a DRAM is increased; it is indispensable in a FRAM. Though the FRAM is a kind of nonvolatile memory devices such as a flash memory device, it has been watched with keen interest due to excellent endurance of more than 1012 cycles, calculating speed of several tens of nsec, a driving voltage lower than 5V, and low power consumption. In order to be a high value-added product for the FRAM, the FRAM is in need of having cell architecture of 1-transistor/1-capacitor like a DRAM cell. Moreover, it needs to enhance the reliability involved in facilities for data retention and iterative read-out/write-in that is compatible with DRAMs, as well as employing a multi-level interconnection process.
In manufacturing high-dielectric capacitors in DRAM or FRAM devices, it is most important to prevent oxidation of a boundary surface of a contact plug connecting an electrode of a transistor and that of a capacitor. It is essential to contact a transistor with a capacitor in the architecture of 1-transistor/1-capacitor. A contact plug made of polysilicon tungsten is normally used. While the annealing is performed in an oxygen environment after depositing a dielectric film in the ferroelectric capacitor, a surface of the contact plug or a boundary surface of the material is oxidized when the dielectric film is exposed to oxygen directly or indirectly through a material that is permeable to oxygen.
The contact plug is connected to a bottom electrode of the ferroelectric capacitor. A platinum electrode, which is resistant to heat and oxidation, is used for the bottom electrode of the ferroelectric capacitor. However, the platinum electrode does not stop oxygen from penetrating. Therefore, the boundary surface of the contact plug connected to the bottom electrode is oxidized by oxygen of the dielectric film or during annealing procedure through the bottom electrode. The nonconductive silicon and tungsten (W) oxide film causes a high contact resistance, and thereby a normal operation of cell is not achieved even when only a part of the surface or boundary surface is oxidized.
Therefore, in forming a ferroelectric capacitor, an oxygen barrier layer is generally needed between the contact plug and the bottom electrode in order to prevent the boundary surface from oxidizing. Many materials have been studied for an oxygen barrier layer. The oxygen barrier needs to be made of a material that is a conductor itself and not oxidized, that keeps the conductivity even though the material has been oxidized, or is impermeable to oxygen, even if it is oxidized. The material for the barrier layer preferably adheres well to a capacitor bottom electrode and a contact plug, and is adaptable to them in forming an ohmic contact.
Recently, precious metals such as iridium, ruthenium, osmium, etc., have been used for the oxygen barrier of the ferroelectric capacitor. For example, referring to Japanese patent publication No. JP1998-242408, it can be seen that a titanium wetting layer, an iridium-yttrium-oxygen compounding barrier layer, and a titanium wetting layer are constructed between a contact plug and a platinum bottom electrode. Further, referring to Japanese patent publication No. JP1998-173138, a multi-level oxygen barrier layer of a titanium ohmic contact layer, a titanium nitridefilm for silicon diffusion barrier layer, and an iridium/iridium oxide/iridium multi-layer are constructed between a contact plug of silicon and a bottom electrode of a ferroelectric capacitor.
The conventional architectures of oxygen barrier layers prevent oxygen from diffusing beyond the barrier layer. However, even though a very small amount of oxygen may enter into the barrier layer, the oxygen that penetrates the barrier layer forms an oxide film in a boundary surface of the contact plug connected with the barrier layer, and thereby contact resistance may be increased and a cell may not operate normally. Namely, it creates problems by increasing the required driving voltage and operating time.
With high integration in semiconductor devices, sizes of the devices and the thickness of barrier layers have been smaller. Thus, there may be a higher probability that the oxygen penetrates into the barrier layer.